ENCePP Guide on Methodological Standards in Pharmacoepidemiology

5.3.2. Case-only designs

Case-only designs reduce
confounding by using the exposure history of each case as its own control and
thereby eliminate confounding by characteristics that are constant over time, as
demographics, socio-economic factors, genetics and chronic diseases.

The self-controlled case series
(SCCS) design was primarily developed to investigate the association between a
vaccine and an adverse event but is increasingly used to study drug exposure. In
this design, the observation period following each exposure for each case is
divided into risk period(s) (e.g. number(s) of days immediately following each
exposure) and a control period (e.g. the remaining observation period).
Incidence rates within the risk period after exposure are compared with
incidence rates within the control period.

Like cohort or case-control
studies, the SCCS method remains, however, susceptible to confounding by
indication, at least if the indication varies over time. Relevant time intervals
for the risk and control periods need also to be defined and this may become
complex, e.g. with primary vaccination with several doses. The bias introduced
by inaccurate specification of the risk window is discussed and a data-based
approach for identifying the optimal risk windows is proposed in Identifying optimal risk windows for self-controlled case series
studies of vaccine safety (Stat Med 2011; 30(7):742-52).